The concentrations of circulating immune complexes (CICs) have been measured in healthy Nigerians aged 6-95 years by the polyethylene glycol precipitation technique. The prevalence of antinuclear antibodies (ANAs) and rheumatoid factors (RFs) was also studied in these Nigerians. Significant positive correlation between CIC concentrations and age was observed; but no sex-related differences. Weakly reacting ANAs plus age-associated increase in ANA prevalence were noted. Positivity rate for RFs detected by latex agglutination was significantly higher (chi 2 = 3.948; P < 0.05) in old subjects (> 65 years of age; 9.4%) compared to younger ones (< 65 years of age; 2.7%). Semi-quantitative Rose-Waaler technique gave 22% RF seropositivity rate in subjects with age > 65 years. The age groups 46-65 years, 26-45 years, and 6-25 years had 13.5%, 7.5% and 2.5% positivity rates, respectively. There was significant positive correlation between RF concentrations and CIC levels; and increased prevalence of RF autoantibody in subjects with CIC (10%) compared with those without (4.7%). Four percent of subjects with CICs, and none of those without, were positive for ANAs. Autoantibodies may contribute to increased prevalence of CICs in old individuals.